The decision by Christopher T. Olivia, MD, to take the chief executive position at West Penn Allegheny Health System of Pittsburgh has delivered a setback to Boston's Caritas Christi Health Care System. Olivia was the top candidate to take over the troubled six-hospital Caritas Christi. In a statement, James J. Karam, chairman of Caritas Christi's board of governors, said the chain would select a chief executive "in the coming weeks."